It is well known that oil wells include various casing and tubing portions disposed one inside the other and opening at the surface at slightly different levels inside a well head, the purpose of which is to support them in their determined position.
The well heads are formed from a number of flanged spools mounted on each other, their purpose being to support a casing or tubing column while at the same time maintaining a hermetic inner and outer seals between one column and the other and between the columns and the surrounding environment.
FIG. 1 shows typical flanging systems of the known art. The two lower systems are used for suspending casing columns, and the upper system is used for suspending tubing columns. Each of the two lower flanging systems has:
(a) two superposed spools 1 having an inner diameter suitable for the casing to be hung 2, and having an internal conicity which is calculated as a function of the weight to be supported; PA1 (b) a set of positioning wedges 3 arranged to support the casing column 2 and calculated as a function of the weight of the column; PA1 (c) a primary gasket pack, the purpose of which is to seal the casings relative to the surrounding environment, and which includes a rubber ring 5 compressed between two iron rings 6 of trapezoidal cross-section which cause the rubber ring to adhere to the outer surface of the casings 2, and to the inner surface of the spools 1 when two spools are clamped together by means of stay bolts or clamps; PA1 (d) a smaller secondary gasket pack 7 housed below the overlying support spool for the next column, its purpose being to form the inner seal for the column using the same elements; PA1 (e) a joint ring 8 compressed between two overlying spools, its purpose being to ensure the inner and outer seal should there be any defect in the gaskets. PA1 (a) 1st cementation stage PA1 (b) dismantling and lifting the BOPS PA1 (c) fitting the fixing wedges for centering the column PA1 (d) lowering the reassembling the BOPS PA1 (e) 2nd cementation stage PA1 (f) dismantling and lifting the BOPS PA1 (g) installing the gasket packs PA1 (h) mounting the upper spool 2 PA1 (i) reassembling the BOPS
The upper flanging system includes the spool 1 and the production cover 9, in which there is disposed a tubing hanger 10 which is internally threaded at its ends. The last tube 11 of the tubing column is screwed to the lower end of the hanger, this column thus remaining suspended from the hanger.
The outside of said hanger is constituted by a cylindrical surface which at its upper and lower ends has a smaller diameter than the diameter of its central part.
An annular rubber gasket 12, a cylindrical ring 13 slidable on said surface and having its lower edge bevelled, and a threaded retention ring 14 are disposed on the lower cylindrical surface. When the hanger is lowered into its seat inside the cover 9 by means of a handling tube screwed to its upper end, it causes the bevelled edge of the slidable ring 13 to rest on a corresponding projection of its seat, and the rubber gasket 12 becomes compressed and expanded outwards, to form the outer seal. The inner seal is formed by a safety valve screwed into the hanger.
Besides the type of suspension for the tubing heretofore described, there obviously exist other more or less sophisticated types, all of which however make their outer seal by means of rubber elements.
The first type of flanging system has considerable drawbacks both with regard to the seal provided by the gasket packs, and with regard to the fitting of the fixing wedges and the centering of the casing.
This is because the rubber rings 5 compressed between the two iron rings are subjected to rapid wear by virtue of the passage of time and the high temperatures, and often cannot resist the high pressures which arise in certain wells. In addition, the positioning wedges and the said gasket packs which are strongly compressed against the outer circumference of the casing can cause it to fracture when, because of the various passages of the rod joints and of the rotation of the drive rod which slides against the inner walls of the casing, these walls become thin to the extent that they become weakened.
Finally, in order to fit the positioning wedges, to centre the casing and to then assemble the gasket packs, it is necessary to dismantle and remove the blow-out preventers (or BOPS) twice, with considerable loss of time and high operating costs.
In this respect, the main operations which have to be carried out for this type of flanging system each time a portion of well has to be cased are as follows:
The second type of flanging system for suspending tubing columns has drawbacks, especially with regard to the outer seal.
This is because at the high pressures and temperatures, the rubber ring and sliding ring become worn and deform rapidly, because of which the seal becomes weakened or becomes completely lacking.